Forced Entertainment have seldom strayed away from performance and into theatre but when they do the result can be galvanising, and a reminder of the wide vocabulary of a company that is 30 this year and still at the top of its game. In Agota Kristof's story of young twin brothers sent during the second world war to live with their grandmother in the Hungarian countryside, they have a story and a text that matches their singular, distinctive voice. Kristof's novel is even arranged in short passages or scenes.

Dressed identically in grey suits, magenta pullovers and glasses, Richard Lowdon and Robin Arthur read the script like young children reading from their own diary, the notebook of the title. They themselves are like a blank slate on which the experience – and horrors – of war are overwritten. There is a fairytale quality: the cottage belonging to grandmother, who is branded a witch by local gossip, is on the edge of a forest. War brings out the cruelty of the local population. Like all children, the boys are natural spies. But they decide to learn from their observations and use what they discover for their own survival. In a world of wartorn chaos they mete out their own moral justice based on absolute need and a total lack of sentimentality.

What's fascinating in the performance is the way that we see the two actors playing the two boys standing before us, but we only hear one voice, a coolly dispassionate one. It's a voice that is constantly probing and constantly troubling. "We have a very simple rule: the composition must be true," say the boys about the contents of their notebook. But in war, truth is one of the very first casualties, and all representations of it are suspect. Who are the real monsters here? This is a knotty evening – not easy, but always fascinating.